We introduce an interesting method to calculate Feynman diagram, especially loop diagram, in QED. The method does not use Feynman parameterization and Wick rotation, instead, it emphasizes the application of Hadamard principal value of integration function to avoid the ambiguities stemming from Wick rotation. Our study shows that the method is more suitable than the standard method in deep time-like region and therefore, the dispersion relations are holden automatically. Furthermore, the method supplies a bridge between the quantum field theory and the condensed matter physics.